The present invention relates to optical films useful, e.g., as polarizers or mirrors, or both, which are combined with a transparent conductor to provide good reflectivity in the infrared region of the spectrum while still transmitting visible light.
Light-reflecting devices based upon multiple polymeric layers are known. Examples of such devices include polarizers made of alternating polymeric layers in which the layers have different refractive indices. Use of thin metal layers, such as silver, or a degenerate semiconductor like indium tin oxide, for electrical conductivity effective in the far infrared spectrum is also known. Such metal layers have been combined with dielectric layers to provide effective visible transmission.
The optical properties and design considerations of birefringent optical films described herein allow the construction of multilayer stacks for which the Brewster angle (the angle at which reflectance of p polarized light goes to zero) is very large or is nonexistent for the polymer layer interfaces. This allows for the construction of multilayer mirrors and polarizers whose reflectivity for p polarized light decreases slowly with angle of incidence, is independent of angle of incidence, or increases with angle of incidence away from the normal. As a result, multilayered films having high reflectivity (for both s and p polarized light for any incident direction in the case of mirrors, and for the selected direction in the case of polarizers) over a wide bandwidth, can be achieved. These multilayered films are combined with a transparent conductor layer, such as silver, to provide broader reflectivity than either the multilayered polymer film or the transparent conductor alone, with the multilayered film providing good near infrared reflection and the transparent conductor providing good far infrared reflection.
Briefly, the present invention includes a multilayered polymer film in which the layers have an average thickness of not more than 0.5 microns. More particularly, in one aspect of the present invention the multilayered polymer film comprises layers of a birefringent polymer, especially a crystalline, semi-crystalline, or liquid crystaline polymer, such as a naphthalene dicarboxylic acid polyester, for example a 2,6-polyethylene naphthalate (xe2x80x9cPENxe2x80x9d) or a copolymer derived from ethylene glycol, naphthalene dicarboxylic acid and some other acids such as terephthalate (xe2x80x9ccoPENxe2x80x9d), having an average thickness of not more than 0.5 microns, and preferably with a positive stress optical coefficient, i.e., upon stretching, its index of refraction in the stretch direction increases; and layers of a selected second polymer, for example a polyethylene terephthalate (xe2x80x9cPETxe2x80x9d) or a coPEN, having an average thickness of not more than 0.5 microns. Preferably, after stretching such multilayered polymer films in at least one direction, the layers of said naphthalene dicarboxylic acid polyester have a higher index of refraction associated with at least one in-plane axis than the layers of the second polymer. The film of this invention can be used to prepare multilayer films having an average reflectivity of at least 50% over at least a 100 nm wide band.
Another aspect of the present invention includes a multilayered polymer film comprising layers of a birefringent polymer, especially a crystalline, semi-crystalline, or liquid crystalline polymer, for example a polyester such as PET polymer, having an average thickness of not more than 0.5 microns; and layers of a selected second polymer, for example a polyester or a polystyrene, having an average thickness of not more than 0.5 microns; wherein said film has been stretched in at least one direction to at least twice that direction""s unstretched dimension. The film of this invention can be used to prepare multilayer films having an average reflectivity of at least 50% over at least a 100 nm wide band.
The multilayered polymer films used in the present invention are combined with a transparent conductor comprising at least one layer containing a metal or a metal compound in which the latter may be selected from the group consisting of semiconductive metal oxides, metal alloys, and combinations thereof. Preferred transparent conductors include silver, gold, aluminum, copper, and indium tin oxide, with silver and indium tin oxide being particularly preferred. The transparent conductor may be deposited on the multilayered polymer film by conventional means, such as vapor deposition, cathode sputtering, and the like, or it may be a separate metalized polymer or glass sheet that is laminated to the multilayered polymer film, such as by a suitable transparent adhesive. The thickness of the transparent conductor layer that is deposited on or combined with the multilayered polymer film is controlled to achieve the desired reflectivity, the actual thickness depending upon the electrical conductivity of the particular metal, metal alloy, or metal oxide used. The resulting transparent multilayer devices preferably reflect a majority of light in the infrared region of the spectrum while still transmitting sufficient light in the visible region of the spectrum to be transparent, and have a shading coefficient of less than about 0.5.